Explaining Blue Stragglers’ Youthful Appearance

Astronomers from the University of Wisconsin-Madison have recently published an article that sheds light over the ‘youthful’ appearance of stars known as blue stragglers. These shiny stars, which seem older than they are, have puzzled scientists for almost 50 years; now, the Wisconsin team shows that blue stragglers increase their mass by stealing mass from nearby stars.
 An artist’s time series shows two stars colliding to form a blue straggler star (Credit: Barry Roal Carlsen)
An artist’s time series shows two stars 
colliding to form a blue straggler star 
(Credit: Barry Roal Carlsen)

Blue stragglers are stars in open or globular clusters that are hotter and bluer than other cluster stars having the same luminosity. One of the reasons they are the focus of many researches is that blue straggler stars appear to violate standard theories of stellar evolution; instead of being on the clearly defined curve in the Hertzsprung-Russell diagram, with their positions determined solely by their initial mass, they behave like they undergo abnormal stellar evolution.

Now, Robert Mathieu and Aaron Geller, astronomers from University of Wisconsin-Madison, claim in the Dec. 24 issue of Nature that blue stragglers steal mass from companion stars and that they sometimes do so by crashing into their neighbors, a scenario once thought far-fetched. “These blue, luminous stars should have used up their hydrogen fuel and flamed out long ago, yet they are still here. By some means or another, they have recently increased their mass, their fuel supply,” says Mathieu.

In their report, Geller and Mathieu show that the mass-gathering ways of blue stragglers conform to all three of the scenarios astrophysicists have theorized. According to the study, the possibility of stellar smashups is greatly enhanced in the star cluster, since binary star systems brush up against one another and swirl into intersecting course orbits – sometimes resulting in collisions.

The study relies on a decade of careful observation of an old star cluster known as NGC 188, situated in the sky near Polaris, the North Star, and located about 6,000 light years from Earth. By observing this cluster the astronomers hoped to learn more about stellar evolution. Their hypothesis is that blue stragglers got bigger in one of three possible ways; all of these possibilities involve companion stars that orbit one another

The first suggested possibility involves two stars in a relatively close binary orbit; one of the stars puffs up into a red giant. In this scenario, the red giant dumps its outer envelope onto its companion star, setting the stage for it to become a blue straggler. The second possibility, offered more recently, is that stars are colliding. While this is a rare occurrence, when binary star systems cross paths, gravitational chaos ensues and the chance is increased. The third option is that a blue straggler is created when a third star brushes up against a binary star system, resulting in the merging of the binary stars with each other, into one more massive star. “In all three scenarios, you end up with more massive stars called blue stragglers,” notes Mathieu.

Mathieu is considered to be an expert on binary stars; studying NGC 188 for almost a decade, he feels quite familiar with most binary systems’ behavior. “These aren’t just normal stars that are straggling behind in their evolution,” he says. “There is something unusual going on with their companions.”

Geller, a UW-Madison graduate student, adds that NGC 188 has a relatively large number and diverse types of blue stragglers, providing the team a good specimen – and therefore they can induce about other binary systems. “Almost certainly these blue stragglers formed separately, and then the two binaries that each were in encountered one another, ejecting two of the stars and leaving behind this truly unique object,” he states.

Another finding the team published is that the blue stragglers are spinning much faster than the average star. Mathieu and Geller hope to use this quality to determine how recently these stars were formed. “People have been trying to find distinguishing properties of these stars for 50 years,” concludes Mathieu. “What blue stragglers are showing us is that life in a star cluster is rarely a lonely existence.”

TFOT has covered other astronomy-related stories, such as the Baffling Supernova, observed by scientists from the Weizmann Institute of Science in Israel and San Diego State University, and Spitzer’s images of a Baby Brown Dwarf, a unique kind of a star. Other related TFOT stories include the proposal of a new theory regarding Galaxies Formation, and the finding of a Ring around a Dead Star, made by NASA’s Spitzer Telescope.

For more information about Blue Stragglers, see University of Wisconsin-Madison’s website.

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